This invention is in the field of persistent switches for superconducting systems.
Advances in the field of superconducting materials and systems have recently enabled the development of important applications of superconductors. One such application is commonly referred to in the art as superconducting magnetic energy storage, or SMES. SMES systems are being used to store energy for high-reliability electrical power systems, where loss of power to critical loads is to be avoided. In the event of a power loss, the SMES system extracts power from energy stored in the magnetic field generated by current conducted through a superconducting electromagnet, and rapidly provides this extracted power to the loads. Because the coil of the electromagnet is maintained in a superconducting state, no resistive losses are incurred in the magnet, allowing for efficient energy storage.
Referring now to FIG. 1, the construction and operation of conventional SMES system 3 for providing backup power to system load 11, by way of power conditioning system 2, will now be described. System load 11 includes the ultimate end use devices (computers, motors, lighting, and the like), and thus serves as the load to conventional SMES system 3. Power conditioning system 2 includes conventional circuitry, such as inverters and transformers, for receiving power from the utility and from SMES system 3, and for distributing, regulating, and applying the received power to system load 11 in the conventional fashion. In normal (i.e., non-failed) operation, system load 11 receives its power from the appropriate utility via conventional power conditioning system 2, as shown in FIG. 1; upon loss of power from the utility, system load 11 will receive temporary backup power from SMES system 3, also via power conditioning system 2.
Conventional SMES system 3 in FIG. 1 includes superconducting magnet 10, which is constructed in the conventional manner as a coil of superconducting wire maintained at superconducting temperatures by cryostat 8. Cryostat 8 is cooled by refrigeration system 9, which is typically powered by utility power. Magnet 10 is part of an electrical circuit that includes power supply 4 and switch 5, both of which are located outside of cryostat 8 in the usual manner. In conventional SMES systems, switch 5 is a solid-state high-current switch implemented by way of a room-temperature semiconductor device. Controller 7 is conventional circuitry that monitors the state of power received and distributed by power conditioning system 2, and that controls switch 5 accordingly.
In operation, magnet 10 is typically energized from power supply 4, with controller 7 maintaining switch 5 closed. Refrigeration system 9 has cooled cryostat 8 to superconducting temperatures by this time. The low impedance of switch 5 in its closed state, relative to the load of power conditioning system 2, keeps any substantial current from being shunted to power conditioning system 2 so long as switch 5 remains closed. The current circulating through magnet 10, which may be on the order of 1 kA, generates a sizable magnetic field which stores the backup energy. Power supply 4, connected in series with magnet 10 and switch 5, serves as a trickle supply to replace power lost by conduction of current through the non-superconducting portion of the magnet circuit (i.e., through switch 5 and the wires external to cryostat 8).
At such time as utility-fed power to power conditioning system 2 is lost, controller 7 senses this condition and opens switch 5. The energy stored in the magnetic field of magnet 10 is then automatically applied to power conditioning system 2 in the form of electrical current, since switch 5 is no longer shunting current from power conditioning system 2.
As noted above, conventional SMES system 3 utilizes a non-superconducting element as switch 5. As a result, significant energy is consumed by the system of FIG. 1, particularly in the replacement of energy by power supply 4 for the resistive losses in switch 5 and the connecting wires. Resistive energy losses are also incurred in room-temperature power supply 4 itself. In addition, because the circulating current is conducted outside of cryostat 8 in SMES system 3, the physical size of the conductors exiting cryostat 8 are quite large and thus conduct significant heat into cryostat 8; this addition of heat adds load to refrigeration system 9 in maintaining cryostat 8 at superconducting temperatures. These factors all add to the cost of operating conventional SMES system 3.
By way of further background, superconducting persistent switches are well known in the art. Persistent switches are typically lengths of superconducting material that can be selectably operated in the superconducting and resistive regions. As is fundamental in the art of superconductors, a superconducting material is in a superconducting state when operated within a window of permissible ranges of temperature, external magnetic field, and current. Persistent switches operate by changing either the temperature, current, or magnetic field of the superconducting material from within the superconductivity window to an operating point outside of the superconducting range, thus normalizing the material (i.e., switching its operation from superconducting to a resistive state).
Conventional persistent switches of the thermal type operate by heating the superconducting material to a temperature above its superconducting critical temperature. One known thermal persistent switch includes a resistive wire wound about the superconducting wire; normalization of the superconducting material is effected by applying a DC current to the resistive wire, heating the superconducting material to above its critical temperature. In these conventional persistent switches, the resistive wire must of course be electrically insulated from the superconducting wire. However, because electrical insulating materials are also generally thermally insulating, the efficiency and speed with which the superconducting wire can be heated in these switches is damped by the electrical insulator. As such, conventional thermally-switched persistent switches do not have sufficiently rapid switching times for many applications, such as SMES systems.
A second type of persistent switch injects an overcurrent into the superconducting material, raising the current conducted thereby to a level above the maximum permitted for superconductivity. The injected current also may induce some amount of eddy current heating in the superconducting wire, which assists normalization of the material. However, overcurrent persistent switches generally require the use of large external power supplies to provide the overcurrent, particularly when the operating current of the superconducting persistent switch is significantly less than the superconducting threshold current, as is typically the case. The cost and complexity of implementing overcurrent mode persistent switches is thus significant, particularly in fast switching applications such as SMES systems.
Magnetically-actuated persistent switches are also known in the art. These switches include an external coil, or electromagnet, for generating a sufficiently high magnetic field to normalize the superconducting material. Of course, both the external coil (of significant size) and driving circuitry must be provided for such switches, increasing the cost and implementation space therefor. In addition, particularly in cases where the operating magnetic field is much below the critical field limit, the size of the external coil can increase to a point at which its response is slow, slowing the switching speed of the persistent switch to a point at which it is not useful for SMES and other fast switching speed applications.